The present invention pertains to a high voltage resistor for outdoor insulating arrangements suitable for preventing pollution flashovers. The resistor is comprised of an insulator body and a resistance material which is connected in series with an insulator. In such arrangements, one or more high voltage resistors and high voltage insulators of any desired design, such as, for example, long rods, post insulators or cap-and-pin type insulators can be used, both for d.c. as well as for a.c. voltages.
The high voltage resistor is intended to prevent flashovers caused by conductive layers, particularly wetted pollution layers, on the surface of outdoor insulators. In the case of such conductive surfaces, initially a so-called pollution-leakage current flows. This current dries the layers at the locations of highest current densities, and so-called dry zones are formed. These dry zones are subsequently bridged over by partial arcs as the result of the non-uniform voltage distribution. If the conductivity of the zones which are still wet is excessive, the partial arcs elongate and flashover occurs at the line-to-ground voltage. Attempts are being made to prevent this flashover by increasing the leakage path with greater overall length for the same insulator profile, or by retaining the overall length and using insulators with a longer leakage distance. However, the use of these two measures is possible to a limited extent only, so that flashovers can still occur in the case of heavier pollution. In the case of very heavy pollution, these measures are not successful. It has therefore been attempted, as shown by British Pat. No. 1,039,193, to provide high voltage insulators with a conductive surface, in order to prevent the non-uniform voltage distribution, which is responsible for the formation of partial arcs. In particular, the semi-conductive glaze proposed therein is intended to prevent the necessary wetting by heating of the insulator. The disadvantage of this solution resides in the fact that high leakage current losses are constantly generated. Furthermore, layers of this type are difficult to produce with the necessary uniformity, thermal stability and aging resistance, especially for large insulators.
Another measure is found in British Pat. No. 1,296,038. In order to prevent surface pollution flashovers, a cylindrical resistor is arranged in series with the insulator. This resistor is dimensioned so that the leakage current flowing over the surface of the insulator remains small and does not exceed a certain value. The resistor required for this purpose must have resistance values within a range of several mega-ohms to one hundred mega-ohms. The disadvantage of this system is that, following the formation of a conductive layer on the insulator, nearly all of the line-to-ground voltage must be taken over by the resistor, since the value of the surface resistance in the case of heavy pollution is very much lower than that of the resistor connected in series. This makes the insulator arrangement very long. Furthermore, the arrangement becomes ineffective if a conductive layer is formed by pollution on the surface of the resistor connected in series, so that it is necessary to mount covers, for example, of a conical configuration to protect the construction from pollution, as shown by the embodiment illustrated in the British patent.